The structure and biological properties of two Moloney sarcoma provirus isolates have been characterized. The integrated form of proviruses, cloned from transformed cell genomic DNA, have been subcloned and molecularly manipulated to determine portions of the viral genome essential for transformation. Both proviruses have terminal repeated sequences (TRS) approximately 600 bp in length, which contain putative RNA transcription control signals. The general structures of the TRS and the integrated provirus are similar to those of some bacterial transposable elements. The transforming activity of MSV has been located within the provirus by direct transfection of cells with cloned subgenomic DNA fragments of MSV. In addition to src, two leukemia derived sequences are required for efficient transformation: 1) approximately 150 bases immediately adjacent to the 5' side of src and; 2) one copy of the TRS, either 5' or 3' to src.